A radio communication system and multicast communication method of this kind have previously been described in Unexamined Japanese Patent Publication No. 2000-32007.
FIG. 1 is a block diagram showing the configuration of a conventional mobile communication system.
In FIG. 1, a router (IGMP compatible router) 3 is connected to a base station apparatus 2, and an IGMP compatible network 4 is connected to the router 3. A plurality of mobile station apparatuses 6-1 through 6-3 are arranged so as to be connected arbitrarily to terminal apparatuses, such as communication terminal apparatuses, 7-1 through 7-3 provided with computer functions.
As shown in FIG. 2, the base station apparatus 2 comprises a radio transmitting/receiving section 11, modulator/demodulator 12, frame composition section 13, and network interface section 15.
As shown by the representative case of mobile station apparatus 6-1 in FIG. 3, each of mobile station apparatuses 6-1 through 6-3 comprises a radio transmitting/receiving section 21, modulator/demodulator 22, frame composition section 23, and terminal interface section 25.
With this kind of configuration, when an IGMP inquiry message arrives at the router 3 from the IGMP compatible network 4, the router 3 transfers an IP packet containing the IGMP inquiry message to subordinate base station apparatus 2.
In the base station apparatus 2, when an IP packet is received by the network interface section 15, a frame signal is composed by the frame composition section 13 by mapping that IP packet onto an L-PDU, which is a fixed-length packet on the user data channel (UDCH).
That frame signal is then transmitted to all mobile station apparatuses using an IGMP inquiry dedicated connection (DUC-ID#1), which is a connection to all mobile station apparatuses 6-1 through 6-3.
That is to say, after being modulated by the modulator/demodulator 12, the configured frame signal undergoes various kinds of transmission processing such as D/A (Digital/Analog) conversion, up-conversion, and amplification (gain control) in the radio transmitting/receiving section 11, and is transmitted as a radio signal from the antenna.
This transmit signal is received by the antenna in each of mobile station apparatuses 6-1 through 6-3, and is output to the radio transmitting/receiving section 21. In the radio transmitting/receiving section 21, the received signal undergoes various kinds of reception processing such as amplification (gain control), down-conversion, and A/D (Analog/Digital) conversion.
After undergoing this processing, the signal is demodulated by the modulator/demodulator 22 and then undergoes frame decomposition by the frame composition section 23. By this means, an IP packet is extracted. This IP packet is output via the terminal interface section 25 to terminal apparatuses 7-1 through 7-3.
When mobile station apparatuses 6-1 through 6-3 receive an IGMP inquiry message contained in an IP packet, a frame signal is composed by mapping an IP packet containing an IGMP report message indicating the multicast IP address for which reception is desired onto an L-PDU fixed-length packet on the UDCH. This frame signal is transmitted to the base station apparatus 2 using an IGMP report dedicated connection (DUC-ID#1), which is a base station connection.
On receiving an IGMP report message, the base station apparatus 2 transmits an IP packet containing the IGMP report message to the router 3 and mobile station apparatuses 6-1 through 6-3.
However, with a conventional apparatus, there is a problem in that, in order to map an IP packet containing an IGMP report message onto an L-PDU, which is a fixed-length packet on the UDCH, there is a need for a dedicated UDCH transmission connection used by all mobile station apparatuses 6-1 through 6-3, separate from the normally used UDCH.
In order to solve this problem, there is a method whereby a connection for IGMP reports is shared by mobile station apparatuses 6-1 through 6-3 by performing random access instead of using a dedicated UDCH. However, with this method also, IP packets containing an IGMP inquiry message or IGMP report message are transmitted and received, and therefore information other than that actually required is transmitted and received. There are consequently problems of redundant information, large overhead, and wasteful use of frequency bands.